Graphite shafts are now used in many products as substitutes for shafts made from rigid materials such as steel, wood or the like. For example, many golf clubs now have graphite shafts. Graphite shafts are favored because they are both light and strong as well as being corrosion resistant.
One method of making graphite shafts is known in which an elongated mandrel is used as a form for making the shaft. Graphite composite material is applied to the mandrel to cover the mandrel, the graphite covered mandrel is then heated to set or cure the graphite composite material to form a solid graphite shaft on the mandrel, and the mandrel is then pulled out of the shaft to release the shaft from the mandrel. This is a cost effective and widely used method of making hollow graphite shafts.
However, the above noted shaft making method has one major disadvantage. Until this invention, it could be used to make only straight graphite shafts. This was a result of having to pull the rigid mandrel out of the graphite shaft requiring that the shaft be straight to allow the rigid mandrel to pass back out of the shaft without fracturing it. In addition, unlike steel or metal shafts which once formed can thereafter be bent in a bending fixture, a straight graphite shaft cannot be bent in a subsequent bending step. This would fracture the formed and rigid graphite shaft at the bend location. Consequently, only straight graphite shafts could be produced with the known shaft making method described above.
There are many applications for which bent graphite shafts would be desirable. For example, graphite shafts used in various types of golf clubs might desirably have a bent lower end to facilitate attachment to the club head while allowing the upper end of the shaft to have the proper orientation relative to the golfer. Accordingly, there is a need in the graphite shaft industry to have an easy, cost-effective and reliable method for making bent graphite shafts.
There has been at least one previous attempt to form bent graphite shafts that was known to at least one of the Applicants. This attempt involved releasing a straight graphite shaft from the forming mandrel before the shaft is fully cured and then bending the lower end of the shaft to a desired angle. This bending step would partially fracture the partially cured lower end of the shaft. Additional graphite composite material would then be wrapped around the bent, but fractured, lower end of the shaft. The shaft would then be reheated to recure the graphite material. When finished, the graphite shaft would have a bent lower end.
However, this previous attempt at providing bent graphite shafts has numerous disadvantages. It involves various additional steps and hand labor. For example, it requires that the mandrel be removed from the partially cured shaft, that the shaft be bent and fractured, that new graphite composite material be reapplied by hand over the bent section, and that the curing process be repeated. This is a time consuming and labor intensive process, leading to an expensive shaft. In addition, given the variations in how a particular shaft might fracture when bent and in how the additional graphite composite material might be applied, there was no way to accurately repeat the bend from one shaft to the next. Accordingly, the amount of the bend could be significantly different from one shaft to the next, leading to various difficulties when using such shafts in other products, such as golf clubs.